1. Field of the Invention
The present invention relates to a high tenacity polyethylene naphthalate fiber with superior processability and drawability, which is produced from a polyethylene naphthalate polymer containing a silica compound. The fiber of the present invention provides a dipped cord having excellent dimensional stability and tenacity.
2. Background of the Related Art
A polyethylene naphthalate fiber having high tenacity, high thermal resistance and excellent dimensional stability is suitable for use as industrial yarns and rubber-reinforcing yarns in products such as tires, belts and hoses, and can exhibit optimal physical properties, particularly for tire cord applications. As the existing tire cord material, polyethylene terephthalate, nylon, rayon and aramid fibers, etc., were mainly used, but the rayon and aramid fibers cause environmental problems and require high production cost, and the polyethylene terephthalate and nylon fibers are relatively inferior in tenacity and dimensional stability at high temperature. On the other hand, a polyethylene naphthalate fiber, which is most frequently used as a reinforcement material of tires, can be produced in a similar equipment to a production equipment of the polyethylene terephthalate fiber and also has high tenacity and thermal resistance, and excellent dimensional stabilities such as high elasticity and low shrinkage, so that it is highly favorable for use as a fibrous reinforcement material of high value-added, high performance tires.
Polyethylene-2,6-naphthalates have higher glass transition temperature, crystallization temperature, melting temperature and melting viscosity, than polyethylene terephthalates, due to their bulky naphthalate units. Thus, to enhance their spinnability upon spinning, i.e., to reduce the melting viscosity of their melt upon spinning, they have been spun at a temperature relatively higher than the conventional spinning temperature (310 to 320° C.) of polyethylene terephthalates.
However, since the spinning at high temperature causes the thermal decomposition of the melt, resulting in a deterioration in drawing processability of the yarn and a significant reduction in intrinsic viscosity of the melt, it is difficult to produce a high strength yarn from polyethylene-2,6-naphthalate (see, Japanese patent laid-open publication Nos. Sho 47-35318, 48-64222 and 50-16739).
Japanese Patent No. 2945130 describes a method of producing polyethylene-2,6-naphthalate fibers with high strength and modulus by controlling the spinning speed and spinning draft ratio and changing the drawing temperature, instead of increasing the spinning temperature. Japanese patent laid-open publication No. Sho 62-143938 discloses a method for synthesizing a polyethylene naphthalate polymer, wherein a thermal stabilizer is used to prevent polyethylene naphthalate from thermal decomposition and thus to reduce the concentration of the carboxyl-terminal groups, such that a polyethylene naphthalate yarn produced from this polymer has high tenacity. However, such methods have a limitation in preventing the thermal decomposition of the polyethylene naphthalate, since the polyethylene naphthalate should be spun at a 10° C. higher temperature than polyethylene terephthalate due to its high melting temperature and viscosity. This thermal decomposition causes the contamination of a spinneret upon spinning and makes the fiber contains thermally decomposed materials, thereby adversely affecting the processability and thermal resistance of the resulting fiber.
Furthermore, upon stretching through a spinneret, the polyethylene naphthalate has a higher viscosity and 30-50° C. higher glass transition temperature than the polyethylene terephthalate and thus has inferior processability during spinning and drawing processes. However, such problems are not completely solved by the prior art.
Thus, the present inventors have conducted intensive studies to solve such problems, and consequently, found that when silica, which is mainly used as an additive for polyester films and was reported in use for the high-speed spinning of some polyesters, pluff reduction and an improvement in dyeing property, is added in the melt polymerization step of polyethylene naphthalate, the resulting polyethylene naphthalate has excellent spinnability upon melt-spinning even at low temperature, and when the spinning draft ratio and drawing temperature of the polymer are optimized based on this excellent spinnability, the physical properties of the resulting fiber can be improved. On the basis of this discovery, the present invention was perfected.